Innovative technologies for information storage are aiming to reach the terabit limit, viz. to write more than a trillion bits on an inch squared. The magnetic hard disk drive (HDD) is today's most widely used mass data storage technique. Densest magnetic storage has been demonstrated recently to have reached an areal density of 100 Gbits per square inch (Gbpsi), using perpendicular recording technology. [1] State-of-the-art industrial production fabricates devices with areal density on the order of 50 Gbpsi. Although the annual rate of increase in the areal density of HDD is 60-100%, it is believed, although not proven yet, that the magnetic technology should break down beyond the 200 Gbpsi limit because of uncertainty in the read/write areal density due to superparamagnetic current effects.
Alternative techniques for mass data storage [2,3,4] have been pursued, whose potential is to write information at terabit density, and with a power dissipation comparable to magnetic storage writing. Scanning probe microscopies (SPM) have been demonstrated already more than a decade ago as useful writing/reading tools. [5] For instance, bits can be represented in the form of topographic indentations or protrusions on a flat surface. The unparalleled resolution, both horizontal and vertical, allows SPM to write 1 bit per square nm, which implies an areal density of 600 Terabit per square inch.
This density is however accessible only on perfect crystal surfaces in ultra-high vacuum, which are of no straightforward technological use. Moreover, a single probe SPM is excessively slow, with best data rates demonstrated of 100 kbit/s in writing, and 1-10 Mbit/s for reading. [6,7,8]
A parallel data storage system based on SPM has been developed in the last decade by researchers at IBM Zürich [9,10]. It is a thermomechanical process operated by an array of cantilevers, termed “millipede”, each of them carrying an independent resistive probe [11]. The resistor can be heated upon appyling a suitable voltage, and an individual “bit” can be written as an indentation of the hot tip in a thermoplastic polymeric film. The read out process is based on measuring the heat loss from the tip to the substrate, which is lesser when the tip is above an indentation. Local heating erases the indentation, so the technology is re-writable. By rastering the polymer film below the array of cantilevers, information can be written and read on a large area, at a data rate which is proportional to the number of cantilevers, but is inversely proportional to indentation time and limited by rastering speed. On these basis, the millipede system could support data rates as high as 1-2-Megabits per second. Power consumption is small (100 mW), due to the small displacements of the storage medium with respect to the millipede. This is compatible with flash memory technology and considerably below magnetic recording. A millipede with 1024 cantilevers was fabricated, and a terabit density demonstrated. [12] The millipede technology has also some drawbacks: i) each tip can only write bits one by one; 2) a percentage of non-working levers leaves un-written areas; iii) the film must be sufficiently smooth to let the passive system of cantilevers to operate without individual adjustements of the tips above the surface.
Other processes based on SPM allow one to write information in the form of dots, rather than indentations. Among the highest areal densities achieved, local oxidation of a substrate by scanning force microscopy [13] has demonstrated the highest areal density with dots 1 nm high, 20-40 nm wide and less than 20 nm apart. However, the dots cannot be erased and re-writing is not possible. This approach can be upscaled to paralell writing by using a multiple source of conductive protrusions, either a “millipede” with conductive tips, or a metallic or metal coated stamp. [14,15]
Novel strategies for information storage technology rely upon multistability. Multistable systems can be controllably switched between different configurations of comparable free energy. Multistability is intrinsically present in molecular and supramolecular systems through a variety of properties (conformations, co-conformations, redox and spin states, shape and dimensionality) which can be influenced by external stimuli (such as mechanical, electric, thermal, light). However, most of these changes manifest themselves only over length scales of, at best, a few molecules and in solution.